This invention relates to reverse flow generators and, specifically, to a redesign of the retaining ring and stator core-end taper to optimize the ventilating flow path between the rotor and stator at the annular air gap exit between them.
The reverse flow concept for generator cooling has been developed since the 1980s. The main advantage to using reverse flow is a characteristic high cooling efficiency, and an uprating capability of the rotor end turns. With the reverse flow configuration, the cooling gas flows directly from the coolers to the rotor without passing through a fan, and the cooling gas absorbs no heat other than a portion of the core losses in the yoke. Hence, it offers cooling gas at lower temperatures not possible from other known ventilation arrangements. However, reverse flow ventilation also results in a longer machine and somewhat greater complexity in the generator end region.
In a reverse flow generator, pressure losses occur at backward- and forward- facing steps, sharp turns, sudden contractions and expansions, and any torturous paths in the gap between the rotor and stator. Among these, a primary pressure drop appears at the annular air gap exit due to the sharp change in the flow area and torturous ventilating flow path. Specifically, in the air gap exit region, a xe2x80x9cbottle neckxe2x80x9d is formed between the retaining ring nose and the stator core-end taper. As cooling gas flows through this region, a significant acceleration of the flow causes a large local pressure drop that is only partially recovered downstream. To increase the effectiveness of generator ventilation systems, it is desirable to eliminate the xe2x80x9cbottle neck.xe2x80x9d
In addition, the ventilating flowrate through the air gap exit in a reverse flow generator is about 60-80% of the total fan flowrate, much larger than that in a forward flow generator where only about 30% of the fan flowrate passes through the air gap entrance. Since the ventilation windage loss WOloss at the air gap exit is proportional to the product of the flow rate Q and the pressure drop xcex94p across it, a small change in xcex94p could result in a large change in Wloss due to the high Q values.
Moreover, the conventional design of the air gap exit in reverse flow generators may create high drag forces against the ventilating flow when it passes through the gap exit, and thus lead to lower generator cooling efficiency.
In accordance with the present invention, the ventilating flowpath at the air gap exit is redesigned to smooth the flowpath and to thereby minimize the ventilation windage loss through the air gap exit. Specifically, the rotor retaining ring is provided with a xe2x80x9csplinexe2x80x9d profile near its nose end and the nose end itself is rounded, to thereby improve the axial cooling flow fluidity and enhance cooling capability. This change alone eliminates much of the pressure drop across the air gap exit by increasing the net flow area and decreasing drag. This aspect of the air gap exit redesign is disclosed in our commonly owned co-pending application Ser. No. 09/551,591, filed Apr. 17, 2000, the entirety of which is incorporated herein by reference.
This re-design of the retaining ring is further combined with a re-design of the stator core-end taper to include a smoother profile that further reduces drag by smoothing the flowpath through the gap exit.
Accordingly, in its broader aspects, the invention relates to a stator core for a stator assembly, the core comprising of an annular body having a plurality of stator bars secured therein, and wherein axially opposite ends of the core are formed with a core-end that approximates a smooth curve.
In another aspect, the invention relates to a rotor assembly including a rotor body and a stator, the rotor body having field coils seated within radial slots formed in the rotor body with end turns of the coils extending beyond opposite ends of the rotor body, and annular retaining rings fixed to the opposite ends of the rotor body and adapted to constrain the end turns against centrifugal forces, and wherein the stator includes a core with stator bars secured therein, the core having core-end tapers at opposite ends thereof that define, in combination with the retaining rings, annular ventilation air exit gaps; each the retaining ring comprising axially inboard and outboard ends, and wherein a radially outer surface at the axially inboard end has an aerodynamically smooth spline shape defined by compound radii that merge into a rounded nose portion, the core-end tapers of the stator core tapering in an axial and radially outward direction in a manner approximating a smooth curve.